A Computer Virus is a malicious software code that is designed to spread from one system to another by attaching itself to a system file of an attacked computer system or Mobile rendering it to malfunction or rendering it's Operating system corrupt.

For example:- An infected Universal Serial Bus (USB) or Pen drive can infect the computer or the Mobile Phone. It not only infects but multiplies itself in to many copies and transfers itself to other computers on the network.

In order to prevent a Computer Virus to attack your system, it is highly recommended that a User should install an Anti-Virus software from a reputed vendor like Symantec/Norton, Kaspersky, Avast, Google Play store, Apple or Samsung etc.,

If you do not have an Anti-Virus installed on your PC or Mobile and your computer is already attacked by the Virus then try to install the new Anti-Virus software downloaded online or purchased through original vendor in the form of a CD/DVD. If the system has totally collapsed. Then it is bad news as all your data on Mobile phone or the PC will be lost.

So it is highly essential to scan a USB, CD or any other devices attached to your system/Mobile to be scanned first to check any hidden Virus.

Viruses could be downloaded on to your system through Internet also. In order to protect your PC or Mobile Phone from online attack, avoid clicking on to suspicious links in e-mails or websites offering murky deals. Install a firewall to block unwanted and suspicious traffic entering your computer.

Outdated Operating systems should be removed from your system and latest OS should be installed for your Anti-Virus Program to function properly and protect your PC or Mobile phone.

Always download your Anti-Virus software from original vendor online. Also download other essential utilities such as Adobe flash player from the Adobe site itself by visiting the site.

Also another grey is peer-to-peer or P2P networking, where online users use bit-torrent technologies such as now defunct Lime-wire to share movies, games, software, and other pirated data. There lies a potential threat of Trojans (hidden malicious codes) presenting itself as a genuine file getting downloaded on to your system.

Downloading files on a Mobile is very much similar to a PC, always see what permissions you are allowing newly installed software/App access to on your device, that is the crucial thing to stop your device being manipulated. If you suspect anything suspicious twice check the credentials of the software again online before continuing with the installation.

Often clean your devices of temporary files, free up disk space, keep your installed programs regularly updated through visiting original vendor sites. Never let your personal device in a public space unattended or hand over it for even a few moments.

Always lock up your PC/Mobile through a Pin/Password or Pattern or Voice or Facial recognition lock, don't modify i.e., jailbreaking.

Wireless security is another area where users have to be cautious connecting to an unsecured network, where any one can log in and install a virus. Use a secure Wireless connection only. Bluetooth devices have potential security problems, like Bluetooth Holes and Bluetooth Bonding, often scan your Bluetooth devices for viruses.

Don't click on suspicious links on Twitter, Facebook, MySpace etc.,

Three basic steps for Android Users :
1. Do not leave 'unknown sources enabled'
2.Avoid third party apps i.e., use only apps from Google Play store.
3. Read App Permissions carefully.

For Apple Users :
it is a Unix based operating system it is less susceptible to attacks.